Added validation for 'default_image_id' field for cluster create

Added unit test for new validation.
Fixed old tests without 'default_image_id' provided.

Fixes: bug #1215359

Change-Id: Iee3864eeac697dbb74a9b0b26d3bf1617412d7e3
Merge "Added validation for 'default_image_id' field for cluster create"